Supervisors: Research Fellow Lauri Jalukse (PhD), Professor Ivo Leito (PhD)
Opponent: Jens Enevold Thaulov Andersen (PhD), Technical University of Denmark
DO content in natural waters is a very important parameter. Recent studies show decrease in DO content in several areas of world oceans. Processes leading to this decrease are not completely understood and it is very important to be able to measure DO content very accurately for studying the dynamics of these processes. Amperometric and more recently also optical oxygen sensors are widely used in DO measurements. These sensors need calibration and therefore solutions with accurate DO concentration are necessary. Oxygen is a very unstable analyte due to its chemical, physical and biological properties. For this reason it is almost impossible to prepare oxygen solutions in ordinary way by dissolving certain amount of oxygen in water. The problem can be solved by using some primary method (i.e. method not needing calibration) that also ensures traceability to SI units. The most reliable primary DO measurement method available is the Winkler titration method. For this method several of factors limiting its accuracy were determined, including the volumetric nature of the classical Winkler method. A number of publications propose different modifications that should eliminate or compensate for these disadvantages. However, before the start of this work there were no publications available that would comprehensively review all the important uncertainty sources of the Winkler method and still a lot of room existed for improving the accuracy of the Winkler method. Most of the publications give repeatability of the results only. In some cases individual uncertainty sources were separately estimated. The method proposed in this work differs from the previously proposed method due to its gravimetric approach, which assures lower uncertainty. Detailed analysis of the uncertainty sources and comprehensive uncertainty estimation were carried out. Experiments for determining the different influence factors were carried out, corrections were determined and uncertainty contributions for accounting these influences were estimated. As a result a detailed uncertainty budget was compiled. This budget is very useful for optimizing the function for getting more accurate measurement results. The optimization was carried out and as a result of this the gravimetric Winkler method modification for determination of DO in water giving the results with lowest available uncertainty was developed.
Key words: Winkleri method, dissolved oxygen, primary method, measurement uncertainty